Do not recommend `WithMessage` instead of `.Be`

[cbersch]

Using WithMessage for checking exception messages might be benefitial in some cases, but not in general.
When I want to check the exact message, this fix shouldn't be suggested.
Currently, this issue is triggered when using one of Be, StartWith, EndWith, Contain on the exception message.

At least for Be it shouldn't trigger.

[cbersch]

I guess, this is subject to individual preferences. Probably best solved by tackling #52